This invention relates to the field of marking devices for placing codes and the like on a substrate. More specifically, it relates to marking devices capable of placing alphanumeric codes, bar codes and other useful indicia on the surface of a product which moves relative to the marking device as, for example, on a conveyor. That device marks on objects moved past the laser beams. Alternatively, the present invention can be employed with a system which scans the beams across a stationary substrate, for example, using moving mirrors. Usually the marking is accomplished using a plurality of lasers having sufficient energy to permanently alter the surface of the product to be marked. One device with which the present invention may be employed is disclosed in U.S. patent application Ser. No. 596,898 assigned to the present assignee which application is hereby incorporated by reference.
Such a laser marking device employs a plurality of lasers, for example, seven. The output beam from each laser is sent through a beam delivery tube and a focusing lens onto the marking location. The seven beams are arranged by means of the lens to form a vertical column of seven beams, each capable of marking a dot onto the passing product. By moving the product transversely past the column, a dot matrix arrangement for printing alphanumeric characters is obtained in a manner well known in this art. By selectively turning on the lasers corresponding to the correct positions for a desired alphanumeric character, such symbols may be marked onto the product.
Typically, a 5.times.7 dot matrix is employed and thus, for each character to be printed, the seven lasers must be turned on and off five times in a sequence which will place dots where required to form the character. Each character is formed by turning on selected lasers simultaneously five separate times (each time being referred to as a stroke) as the substrate of the article to be marked moves past the laser lens. In printing characters in this manner variations in laser energy output cause degradation of the print quality. Some dots are only partially formed or are missing entirely. Others are too large. This variation in laser output energy can result from: (1) variation in nominal laser output power, (2) turn on delay, (3) turn on and turn off time constants, (4) energy density and beam shape variation. Indeed, efforts to match seven lasers so that their effective output power is essentially equal would be prohibitively expensive and perhaps impossible as power output can change over time.
The present invention discloses a method and apparatus for equalizing laser output by controlling the electrical drive signals for each laser. The result substantially reduces the effect of laser performance variation on print quality. The control takes several forms. First, the relative energy output of each laser is determined and its on time is adjusted by appropriately weighting the on time of each laser relative to the weakest laser beam. Alternatively, the beam weighting can be relative to other references, such as average beam energy or even an independent reference value. The use of variable width "on" time pulses for the lasers can, however, introduce nonlinearity in the formed characters. To avoid or minimize this problem, all of the "on" pulses are centered about a selected time axis. Thus, the combination of weighted "on" pulses coupled with pulse centering provides a significant improvement in overall print quality.
For high stroke rates an additional problem arises from the inability of the lasers to cycle fast enough. This problem is reduced by selectively preionizing the lasers to reduce turn on delay to permit significantly higher stroke rates while maintaining marking quality. Various pre-ionizing schemes may be employed depending upon the desired application.
It is accordingly an object of the present invention to provide an improved method and apparatus for equalizing the power output of the individual lasers employed in a laser marking system.
A further object of the invention is to provide a method and apparatus for varying the "on" pulse width of each laser in such a marking system to substantially equalize the energy outputs thereof.
Another object of the invention is to provide a method and apparatus for centering the "on" pulses used to trigger the lasers to insure vertical linearity of the energy dots produced therefrom for marking a moving substrate.
A further object of the invention is to provide a method and means whereby high stroke rates may be obtained by pre-ionizing the lasers to reduce turn on delay.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.